1. Field
This invention relates to compound archery bows. It is specifically directed to an improved eccentric wheel for such bows.
2. State of the Art
Compound archery bows have been well known for many years. An early patent descriptive of such bows and their mode of operation is U.S. Pat. 3,486,495. Such bows are generally characterized by "let-off" leveraging devices carried at the distal ends of the limbs. These leveraging devices are usually referred to as wheels or pulleys, although they may take various forms, including some with other than circular cross-sections. They are commonly referred to as "eccentrics," because they characteristically are pivoted around an axle located off center with respect to their perimeters.
The eccentrics carried at the tips of opposite limbs are interconnected by cables and bow string. Although various arrangements are possible, a typical arrangement includes two cables, each of which is anchored at some point on a limb and stretched accross the handle, around at least a portion of the perimeter of the eccentric carried by the opposite limb. The free ends of the cables are fastened to respective opposite ends of a string, sometimes referred to as a "central stretch". Lengths of cable are thus positioned between the string and the handle of the bow. It is important that the string be positioned with respect to the cabling (at least at fired condition), to provide adequate vane clearance. Vane clearance between the string and cables must be sufficient to avoid interference by the cables with an arrow launched by the bow. Such clearance has been provided in various ways. Sometimes, especially when narrow eccentrics are used, structures (cable guards) are carried by the handle or limbs of the bow to hold the cables out away from the plane of the string in the vicinity of the handle. In other instances, wide eccentrics are used, and the cables are located to one side of the eccentrics while the string is located at the other side of the eccentrics so that they are spaced as they cross the handle.
A common problem associated with compound bows (because of the necessity for maintaining vane clearance) is the buildup of torque in the bow limbs as the string is pulled back to full draw position. This torque results from the relatively large buildup of force in the cable system of the bow compared to that of the string. These forces are translated to the axle of the eccentric. They oppose each other in that they are translated to opposite sides of the midpoint of the axis, but they do not balance each other. As a consequence, the axis tends to rotate, thereby tending to twist the bow limb. That is, a bending movement is imparted to the limbs of the bow. A recent innovation has been to provide a spiraled groove on the eccentric so that as the string is pulled to full draw position, and the eccentric pivots, the wound cable migrates across the pulley to near the central location of the string. The force of the cable system is thus applied close to the midpoint of the axle, thereby reducing the bending moment of the limbs at full draw. When an arrow is launched and the string moves towards the handle of the bow, the eccentric pivots back to its static position, and the spiraled groove carries the cable back out to the edge of the pulley.
Eccentrics have been produced in various configurations to achieve certain special characteristics. For example, "side-by-side" pulleys have separate grooves to accommodate the "string" end of the cable and the "take-up" end of the cable. The cable passes through the interior of the pulley to gain access to both grooves. "Side-by-side" pulleys locate the string and cable forces at set distances on opposite sides of the midpoint of the axle. In this way the bending moment at full draw may be fixed within a known limit, while maintaining vane clearance. The limbs may then be strengthened on one side to compensate for the remaining bending moment imparted to the axle at full draw of the bow. This eccentric style develops relatively high speed compared to another popular type, the "step-down" pulley. "Step-down" eccentrics have a "take-up" groove of smaller diameter than the diameter of the string groove. In effect they provide a lower "gear ratio" which reduces the force developed in the cables at full draw. This reduced force effects a correspondingly reduced bending moment at the limb.
Because of the bending moments developed in the limbs at full draw, it has heretofore been necessary to provide different structural members to serve as upper and lower limbs, respectively. If an upper limb were substituted for a lower limb, the structural expedients incorporated to resist the bending moment of the upper limb would be exactly the reverse of what was required for the lower limb.